tfm_ns_interface.c is intended to be overriden by clients to support
different targets. We copy this file from upstream into the mbed-os
platform library. We also have a specific "strong" overridden version
for the NU_M2354 target, which is located in its target library.
Previously the implementations in the platform library were decorated
with __attribute__(weak), and we provided a strong definition for the
NU_M2354 target. This worked fine because of weak linking, the linker
will pick the first "strong" definition and use that, avoiding any ODR
violations. However, upstream have removed __attribute__(weak) from the
function definitions, which caused multiply defined symbol errors when
trying to build the NU_M2354 target.

To work around the above issue, we remove the common definition in the
platform library; instead we copy the file to the Musca B1 and Musca S1
target libaries. This means the appropriate tfm_ns_interface.c is only
included in the build when compiling for the specific target which uses
it.